The technique for detecting a desired signal light (a light to be detected) is a basic and important element in various systems in which light is used, such as vital observation system, sensor system, security system, laser radar system and has a huge influence on their performance. In particular, detection technique with high speed and high sensitivity is widely required.
It is necessary to perform photodetection with high speed in order to realize precise observation, since the condition or shape of a biological body varies momentarily in the vital observation. There is also an upper limit for amount of light with respect to illumination light or excitation light with which a biological sample is irradiated, since the biological body is easily damaged by the light irradiation. Therefore, light signal which can be obtained from the biological body is generally weak. For these reasons, photodetection technique with high speed and high sensitivity is widely expected in the vital observation in which light is used.
Typical light elements used at present include PMT (Photo Multiplier Tube), APD (Avalanche Photo Diode), PD (Photo Diode). In PMT and APD, electron multiplication takes place within a detection element, so that photodetection with high sensitivity is realized. In the case of PD, distinctly high response speed can be realized, but a signal is generally amplified by means of an electric amplifier, since electric amplification function is not provided within the detection element. That is to say, the signal is amplified electrically for each element of PMT, APD, PD, so that sensitivity is improved.
Typical two-dimensional detectors include a CCD (Charge Coupled Device), a CMOS (Complementary Metal Oxide Semiconductor), EM-CCD (Electron Multiplying-CCD), an EB-CCD (Electron Bombardment-CCD) and an I-CCD (Intensified-CCD). If a weak light is detected by the CCD or the CMOS, it is necessary to arrange an electric amplifier downstream thereof, as in the case of APD. The EM-CCD and the EB-CCD have a function of electric amplification, as in the case of the APD, so that high sensitivity is realized. For the I-CCD, II (Image Intensifier) is arranged upstream of the CCD. The II temporarily converts an incident light signal into an electric signal for the electric amplification in a MPC (Micro Channel Plate) integrated in the II and then allows the amplified electrons to impinge on a fluorescent plate to convert the amplified electric signal into light again. The output light from the II is converted into an electric signal at the CCD. Thus, the photodetection with high sensitivity is also realized at the I-CCD by the signal amplification at an electrical step.
Photodetection method in which light intensity is converted into electric current by means of the photodetector device as described above is broadly used. This light detecting method is called “a direct detection method”. In the direct detection method, thermal noise, electric amplification noise or excessive noises are the main factor for noise. These noises are greater than shot noise, and therefore, photodetection with ultimately high sensitivity is considerably difficult.
As one of techniques that allows photodetection at high speed and with high sensitivity, optical heterodyne detection technique is broadly used. The conventional optical heterodyne detection technique is a photodetection method in which interference effect with the local light (local oscillation) having an optical frequency slightly different from that of the light to be detected is used and the light is detected with high sensitivity by sufficiently increased intensity of the local light. If the local light has sufficient intensity, ideal photodetection of a shot noise limit is possible by using a high speed electric circuit, and thus, both high speed and high intensity of the photodetection are realized. In this case, for the signal light and the local light, light in temporally and spatially stable interference condition is generally used.
As a method for temporally stabilizing the interference condition, the following two methods are mainly used. In the first method, an output from the same light source is demultiplexed and each demultiplexed light is used for the signal light and the local light. In this case, due to the demultiplex of the output from the same light source, relative delay time until the signal light and the local light are multiplexed is shorter than the coherence time of the light source. Thereby, the interference condition between the signal light and the local light are temporally stabilized. The light frequencies of the signal light and the local light are configured to be slightly different by using an optical frequency shifter and the like. This method has been used for a long time, since the stable interference condition can be realized relatively easily (See, for example, Patent document 1.).
In the second method, two independent light sources are used, in which the line width of the optical spectrum is significantly narrow (purity of optical spectrum is very high) and the optical frequency is stabilized with high precision. Here, the line width of the optical spectrum is smaller than the bandwidth of the electric circuit part including the photoelectric conversion part. The two independent light sources are used for the signal light or the local light, respectively. In this case, the optical frequencies of the signal light and the local light are configured to be slightly different. Conventionally, this method has been realized only with difficulty due to the technical restriction. However, due to the recent technical development, the purity of the optical spectrum is so high that the line width of the optical spectrum is in a range of kHz and a laser having optical frequency stabilized with high precision is available, so that a relatively stable interference condition can be also obtained at present by using the second method.
It is necessary to use light with high spatial coherence for each of the signal light and the local light, and spatial mode distribution should correspond to each other in order to stabilize the spatial interference condition. For this reason, a spatial mode filter such as a confocal optical system or an optical fiber is used on the signal light path. Thereby, only the signal light component keeping an interference condition spatially stable can be extracted. Consequently, the interference condition is spatially stabilized.
Patent document 1: JP 2890309